This invention relates to doctor blade assemblies used for metering a liquid such as an ink or a coating material onto the surface of a rotating transfer roller.
Doctor blade assemblies are commonly employed to meter a liquid, such as an ink or a coating material, onto a transfer roller for ultimate disposition on the surface of a flexible substrate that is to be printed upon or coated. Conventionally, a doctor blade assembly includes a small, elongated reservoir of a length at least equal to that of the transfer roll. An elongated opening extends the length of the reservoir and on opposite sides of the opening, doctor blades are located. The liquid to be transferred to the transfer roll is placed in the reservoir and the transfer roller is rotated in engagement with the doctor blades. Its surface receives the liquid to be transferred from the interior of the reservoir. As the wetted surface of the roller passes the first doctor blade, the same causes the film of liquid on the surface to be made uniform for ultimate transfer to a substrate. As the roller continues to rotate, the second doctor blade is encountered as the roller surface is about to reenter the reservoir. This doctor blade is frequently called a containment blade and allows ink or other liquid remaining on the roller to reenter the reservoir while preventing the liquid in the reservoir from splashing outwardly thereof.
In conventional doctor blade assemblies, the two doctor blades are separated by 90xc2x0 or less when measured angularly within the reservoir. Moreover, both oft he doctor blades engage the surface of the roller with approximately the same contact pressure which can cause the liquid being transferred to puddle on the side of the containment blade that is exterior of the reservoir and cause leakage problems.
Leakage of the liquid being coated on the transfer roll is also particularly vexatious in that periodic clean up is required and adds to the cost of the operation employing the doctor blade assembly.
The leakage problem in conventional doctor blade assemblies is accentuated by the fact that in such conventional assemblies, seals at the ends of the rolls engage the cylindrical surface of the roll, in an attempt to prevent all flow of ink to the sides of the roller. Friction thereat generates heat which tends to dry the ink which congeals on the seal and fouls the same, thereby disabling the seal with the consequence that leakage increases with the use of a particular set of seals necessitating frequent replacement of the seals if the leakage is to be avoided.
So-called xe2x80x9cghostingxe2x80x9d is a common problem with the use of conventional doctor blade assemblies and printing applications which is highly undesirable.
The present invention is directed to overcoming one or more of the above problems.
It is the principal object of the invention to provide a new and improved chambered, doctor blade assembly. More specifically, it is an object of the invention to provide such a doctor blade assembly wherein xe2x80x9cghostingxe2x80x9d problems are minimized or eliminated and/or leakage from liquid contained within the chamber is minimized.
An exemplary embodiment of the invention achieves the foregoing object in a structure including a generally cylindrical roll mounted for rotation about an axis and adapted to engage another roll. A chamber is provided which partially surrounds the roll and is generally C-shaped in cross section to define an opening extending parallel to the roll axis through which the roll is accessible to engage another roll. A pair of blades are mounted on the chamber near respective sides of the opening and have edges extending into the chamber and parallel to the axis and engaging the roll.
According to one aspect of the invention, the chamber is in two segments with the segments being mounted for movement relative to each other so that at least one of the segments may move toward or away from the axis of the roll.
According to another aspect of the invention, the edges of the blades are angularly spaced from one another by about at least 180xc2x0 as measured in the chamber.
According to still another aspect of the invention, the roll has a blade engaging surface and the doctor blade edges have a length just greater than the length of the blade engaging surface and extend just past opposite ends of the blade engaging surface.
In a preferred embodiment, foam seals are located at opposite ends of the roll and the doctor blades have ends partially entering the foam seals.
According to still another aspect of the invention, one of the doctor blades is upstream of the other doctor blade in the direction of rotation of the roll and engages the roll with about twice the contact pressure or more than that of the other doctor blade.
According to still another aspect of the invention, the chamber is generally partially cylindrical with a cylindrical axis. The cylindrical axis is parallel to, but offset from, the axis of the roll so that the roll is closer to an interior wall of the chamber at a location generally oppositely of the opening than elsewhere within the chamber.
In still another exemplary embodiment of the invention, the interior wall of the chamber has axially extending grooves.